drm/atomic: add drm_atomic_add_affected_planes

This is a convenience function to add all planes for a crtc,
similar to add_affected_connectors. This will be used in
drm_atomic_helper_check_modeset, but drivers can call it too
when they need to recalculate all state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Amend kerneldoc a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>